FIG. 1 illustrates two DECT (digital European cordless telephone) communication systems interworking with a GSM (global system for mobiles) communication system. The DECT systems each comprise a fixed part 1,2. Each fixed part comprises a cluster control fixed part (CCFP) 3,4 and radio fixed parts (RFP) 5,6,7,8, each of which is connected to the respective CCFP. The CCFPs are connected over an A-Interface 9 to a GSM mobile service centre (MSC) 10. DECT portable part (PP) handsets 11,12 can communicate wirelessly with the RFPs when they are in range. When a portable part moves from the range of one RFP (the "old" RFP) to the range of another RFP (the "new" RFP) there is a need for handover of the wireless communications with the portable part. When the handover is between two RFPs that are under the control of the same CCFP (handover 13 in FIG. 1) this can be handled by the DECT system in a procedure called internal handover, with data loss protection. When the handover is between two RFPs that are under the control of different CCFPs anchored by the MSC (handover 14 in FIG. 1) the procedure is known as external handover.
Document [14] cited below defines the implementation of bearer services. This takes as its basis that in the DECT/GSM interface the GSM error correcting protocol RLP is terminated to the interworking unit (IWU) of the DECT fixed part, and the DECT air interface (based on the C data profile) provides its own error correction over the DECT air interface by re-transmitting the MAC (medium access control) layer protocol and by the DLC (data link control) layer re-transmitting the LAPU protocol. This protocol model is illustrated in FIG. 2, where block 15 represents the DECT portable part, block 16 represents the DECT fixed part and block 17 represents the GSM mobile switch centre interworking unit. Thus, in this standard implementation the RLP and LAPU protocols function independently; the RLP protocol operating over the A-interface (18 in FIG. 2) between GSM and DECT and the LAPU protocol operating over the DECT air interface (19 in FIG. 2). The LAPU protocol runs all the way from the CCFP to the PP, so situations where data might be lost in DECT internal handover can be taken care of either by the DECT MAC layer error correction or by the LAPU. However, error correction in the event of external handover is unclear.
In this document the protocol used for the provision of GSM non-transparent service is referred to as the GSM A-interface LAP protocol. A current example of this protocol is RLP but other protocols (such as V.120) could be used in the future. In some situations in this document the RLP has been used as an example.
In the past, specifically for DECT/GSM interworking have not taken care of external handover. However, there is a possibility that data could be lost if external handover takes place whilst data is being transferred. This is because the RLP and LAPU systems function independently. The MSC receives acknowledgement (sent by the DECT fixed part) of a sent data frame before the frame has reached its final destination (the portable part). If external hardover has occurred and the frame has been sent to the wrong CCFP then it cannot be sent to the portable part.
FIG. 3 illustrates the situation where in a data service--in which the system carries signals representing data that has been provided to the system in a digital form by a user of the DECT portable part, or that is to be provided by the system to that user in a digital form--data frames (I-frames) are being transferred in the mobile terminated direction (i.e. in the direction from the MSC to FP) and then, using the DECT LAPU, over the DECT air interface. The first I-frame is passed at 20 from the interworking function of the MSC 21 to the first fixed part (FP1) 22. FP1 acknowledges the received frame at 23 and then forwards the data at 24 using DECT LAPU to the portable part 25. The PP acknowledges the data at 26. If an external handover of the PP from FP1 to FP2 (at 27) then occurs the FP1 will acknowledge (at 28) the next frame to the MSC but will not be able to send it to the PP. And FP2 cannot resend the data to the PP because it does not have the frame. In addition, in the IWF the GSM LAP send window has been incremented after successful I-frame acknowledgement and the data has been lost from the RLP buffers.
FIG. 4 illustrates the situation of mobile originated transmission. In this case no data loss can occur because the PP can re-transmit the data (at 29) to FP2. However, the PP has to be able to continue the same LAPU connection over the new U-plane link, so the interworking unit of FP2 has to be able to know the status of the state variables (the send and receive counters) of the old link.
Thus the greater problem is encountered when data is being sent towards the portable part (PP) during external handover.
One aim of the invention is to address this problem.